I think its a bad idea for instructors to date student/trainees. There is an inherent power differential in the relationship. Although that does not necessarily suggest "undue influence" it will call into question the instrutor's capacity for unbiased evaluation of the student's performance. It certainly did in one dojo I attended.

I wouldn't question two non-instructors or two instructors (equivalent ranks) hooking up romantically.

I would however suggest that any aikido romeos ask themselves what they are on the mat to do. I only have a few hours a week I can train, and wooing a potential date is a poor use of that time.